A logo for pornographic website Pornhub is featured on the lower left of the image.

"I was shocked. It was a very explicit image of an unnatural, painful looking sex act displayed uncensored and without warning," an Instagram user told Newshub after discovering the image.

"It grossed me out, but I'm a fully grown adult. What worried me was that people of all ages use Instagram and a child could've easily come across that in their timeline unwittingly, just as I did."

While the shocked Instagram user didn't want to be identified, Newshub verified the post was actually in their timeline.

The pornographic T-shirt was one of five items advertised in the RageOn! post. The others linked through to an online store where the displayed items could be purchased, but the link to the explicit item went to a page displaying the message 'this item can not be found'.

A sponsored post is an advert a company pays for on Instagram to promote their content across a variety of users' feeds, without the users having to follow them.